And I mumbled once about wanting an easy way to retrieve information from the metadata in the epub ....
Two tedious workarounds I use occasionally:
To just look at the metadata, open the book viewer, the "i" button displays the metadata from inside the epub --- but you can't select & copy
To grab a piece of it:
As dwanthny described -- set metadata for book from selected format in the Edit Metadata window -- then select the desired comment/whatever, copy to clipboard, click "cancel", re-open the edit window, paste, save.
Too tedious to use often, but does the trick.